CYTOPLASMIC CARBONIC-ANHYDRASE-II OF RAT COAGULATING GLAND IS SECRETED VIA THE APOCRINE EXPORT MODE

Two different pathways for protein secretion are described for epithel ial cells of rat coagulating gland and dorsal prostate: the classical merocrine and the alternative apocrine release mode. Apocrine-secreted proteins are synthesized on cytoplasmic polyribosomes and are subsequ ently exported in protrusions on the apical cell surface (aposomes). I n this article we report the identification and purification to homoge neity of a 29-kD protein from the secretion of rat coagulating gland. N-terminal amino acid sequence analyses revealed 100% identity to rat brain carbonic anhydrase II (CAH II). In addition, the 29-kD protein s howed CAH enzyme activity. On Western blot analysis, a polyclonal anti -CAH II antibody raised in rabbit reacted specifically with the rat an d human but not bovine CAH II isoforms. Immunohistochemical studies on rat coagulating gland showed strong labeling for CAH II protein in ap osomes. Immunoelectron microscopy confined CAH II protein to the cytop lasm and aposomes, whereas no staining was visible in the compartments of the classical merocrine route, the endoplasmic reticulum and Golgi apparatus. The resident cytoplasmic protein lactate dehydrogenase, ho wever, was not found in the secretion. Taken together, the morphologic al and biochemical data clearly indicate that cytoplasmic CAH II from rat coagulating gland is specifically selected and then secreted via t he apocrine pathway.